High efficiency lubricants generally offer lower friction across a wide range of temperatures and conditions. Friction can result not only from surface contact but also from the presence of viscous medium between the mating surfaces of mechanical components. At a given temperature under relatively low load or high speed conditions, two contacting surfaces are separated by a full lubricant fluid film and the resulting friction is referred to as hydrodynamic friction and is mainly determined by the viscosity of the lubricant. In a hydrodynamic lubrication regime, lower lubricant viscosity leads to higher energy efficiency. On the other hand, under high load at low speed or low viscosity conditions, two contacting surfaces will be rubbing against each other and friction is determined by the friction coefficient of the chemical film formed at the two surfaces. This lubrication regime is referred to as the boundary lubrication regime. The lubrication regime in between the two mentioned is referred to as the mixed lubrication regime.
Thus, an ideal lubricant will exhibit a high viscosity at its highest operating temperature to avoid surface contact while exhibiting a relatively low viscosity at the rest of the operating temperature range in order to minimize friction. For a lubricant operating between 100° C. to 150° C., the preferred base fluid would have a high KV150/KV100 ratio. In the event surface contact does occur under high load and low speed conditions, the ideal lubricant will also form a chemical film with a low friction coefficient.
Attempts have been made to use conventional lubricants, such as Groups I, II, III, IV, and V base stocks, in high-temperature applications, such as in high-performance motors and engines. Many conventional lubricants, however, cannot maintain sufficient film thickness at high temperature (e.g., 150° C.) to provide protection in areas like journal bearings while maintaining low hydrodynamic friction at lower temperatures (e.g., 100° C.). Thus, it is highly desirable to have lubricants with viscosities at high temperatures as close to that at low temperatures as possible.
One means of addressing lubrication performance at high temperatures is selection of lubricant base stock. It is difficult to select a conventional lubricant base stock that provides both sufficiently high viscosity at high temperatures and low viscosity at low temperatures. Conventional high viscosity base stocks may provide sufficiently high viscosity at high temperatures but may be too viscous at low temperatures. Conventional low viscosity base stocks may provide sufficient fluidity at low temperatures but provide insufficient viscosity at high temperatures.
A second approach is to improve viscosity-temperature response by adding a polymer to the lubricant formulation. Such polymer is called a viscosity modifier or viscosity index improver (VII). The function of a polymeric viscosity modifier is to increase the high temperature viscosity without significantly increasing the low temperature viscosity. The resulting viscosity-temperature relationship is determined by the base oil viscosity-temperature relationship and the chemical structure of the polymeric viscosity modifier.
Another means of addressing lubricant performance at high temperatures is to employ friction modifying additives, such as molybdenum dithiocarbamate (MoDTC) or glycerol mono-oleate (GMO) in boundary lubrication conditions. However, such friction modifying additives degrade in performance over time. In addition, wear might result if the surfaces are not sufficiently separated by an oil film, despite the presence of a friction modifier.
It would be desirable to have a lubrication system that provides effective performance at high temperatures. It would be desirable to have a lubrication base stock that provides sufficient viscosity at high temperatures yet provide sufficient fluidity at low temperatures. It would be further desirable to have lubrication base stocks that provide such performance without the need for friction modifying additives.